A legacy index measures the number of releases a software product has until it becomes legacy. It can be used to measure and communicate design quality: the more releases on a code base before change becomes difficult and it gets demoted to a legacy system then the better the design.
The target should be a high legacy index or to reach Legacy Max. An application that becomes legacy after its first release would be a Legacy Zero app. The lowest index is awarded to applications which become legacy before they are even released: Legacy minus One!
Applications can still have further releases once they've hit their legacy index: these releases would be identified by consisting mainly of bug fixes and only minor changes, this would be called "pushing the legacy index" as each release would not increase the index. Also an application may be able to reverse its legacy status by refactoring it to a point where change is again possible.
By measuring and plotting an applications legacy index against time you can clearly see the health and state of an application. If it has a steady and consistent incline then you know that the design is sound and the application is healthy but if the index has a trend of slowing or is continuously pushing the legacy index then it is clear that there is a problem.
Though quite humorous legacy indexes may have a genuine use in describing the state of an application, you can simply say "this is a Legacy Zero app" or "it's hit its legacy index" and all is clear.